If unionization were good Flint wouldn't have been in such a poor state it could lose clean water access. What is true is auto jobs didn't leave the US, they left Michigan, primarily for right-to-work states. The same story is true for most of the rust belt; a lot of heavy industry jobs didn't necessarily leave, they just moved to Texas or the south.

Also unions didn't get rid of 16 hour days, market competition and regulation did that. Private industry unions have been consistently behind the private market in terms of benefits. The past you're talking about never existed.